The present invention relates to a method for removal and/or prevention of formation of salt deposits in critical sections of a process plant, in which nitrogen oxide-containing gas is transported, cooled, and possibly dried by removal of condensate and compressed. In the gas of such a plant might be maintained a water vapour pressure which lies above the critical vapour pressure at the existing temperature for the salt deposits.
For catalytic combustion of ammonia to nitrogen oxides, it has been found that there will always be a certain penetration of uncombusted ammonia which proceeds in the process together with the nitrous gas. Especially during start up of the combustion furnaces the penetration of ammonia may be substantial. Ammonium salts will be formed during cooling of the gas, especially ammonium nitrate, which might crystallize and form deposits in the apparatus.
However, the formed salts are hygroscopic and easily soluble in water. When the relative humidity of the gases is higher than the critical humidity of ammonium nitrate, the ammonium nitrate will be present in the form of a solution and will not crystallize and form deposits.
The gases from the combustion unit are cooled before compression. When ammonia is combusted, water vapor is formed and removed from the gas as condensed water vapour during the latter part of the cooling step. The cooling is usually carried out by indirect cooling against cold surfaces such that the gas from the cooler has a dew point which is somewhat lower than the temperature of the gas. Accordingly, the gas will be reheated a few degrees because of the oxidation of NO to NO.sub.2, between the outlet of the coolers and the compressor, according to equation: EQU 2NO=O.sub.2 =2NO.sub.2 H=-228.6 kJ
Because of this temperature rise and a certain admixture of secondary air, the relative humidity of the gas will decrease between the cooler and the compressor.
The problems which are caused due to the formation of salt deposits in the above mentioned plants have been known for a long time. The salt deposits can lead to reduced production capacity, increased energy consumption and safety risks. The rotors of the compressor have been found to be exposed to salt deposits which have led to unbalance and breakage of the rotor blades or other compressor damage.
It has been common practice to inject water into the nitrogen oxide-containing gas in order to remove salt deposits. The compressors are equipped with several nozzles for intermittent water injection during operation. The interval between each washing might be varied from 4-36 hours and the washing time can be varied between 10-30 minutes. Normal addition of water during the washing step is in the range 0.015-0.04 kg/Nm.sup.3 nitrogen oxide gas. Some nitrogen oxide gas compressors also have continuous water injection in the gas in addition to the discontinuous washing.
Usually water is also supplied continuously to the sealing system of the compressor in order to prevent salt deposits. This addition also ends up in the product either by evaporation into the nitrogen oxide gas or by drainage as condensate. Thus there are substantial amounts of water which are necessary to avoid salt deposits. Such addition of wash water into the nitrogen oxide gas or condensate is however strongly unwanted as the water must be compressed and because of the resulting corresponding reduction of the process water to the absorption system. This will also reduce the absorption effectiveness or the maximum obtainable product concentration.
Washing with water will often not give the required cleaning for retaining maximum capacity in spite of the lengthy injection of water into the compressor. The reason for this is that the injected water droplets will not moisten all the surfaces where salt deposits are present because of their inertia. In some compressors the accumulation on these surfaces will be so large that periodic water injection during reduction of the speed of the compressor are necessary in order to obtain maximum capacity.
Injected water droplets can also give substantial erosion in the compressor, especially on the rotor blades in axial compressors and around the pin holes in centrifugal compressors.
There further is known a method for removal of such salt deposits by supplying water vapour in such amounts that there is established a water vapour pressure which lies above the saturation pressure of the salt deposits at the existing temperatures to the nitrogen oxide-containing gas which has been dried during the cooling step. This method is described in U.S. Pat. No. 4,295,895. However, it has been found that even though the method according to this patent is efficient, for certain types of plants increased investments and operation expenses are required. Especially in older plants in which inexpensive steam is not available, this method might be less attractive from an economic point of view.